In-depth proteomic analysis of rice embryo reveals its important roles in seed germination

Seed germination is a complex physiological process that allows the seed embryo to grow and develop into a photosynthetic organism. The two major constituents of rice seed include the embryo and endosperm, with embryo being of much significance despite its small size. In this study, we conducted a systematic proteomic analysis of the embryos dissected from rice seed at different stages of germination through a combination of gel-based and gel-free strategies. In total, 343 differentially expressed proteins were identified. Among them, 191 were decreased and 152 were increased in terms of expression levels. All these proteins could be sorted into 11 functional groups based on MapMan analysis. Some starch biosynthesis-related enzymes such as starch branching enzyme, granule-bound starch synthase 1 and starch synthase increased during the early stage of germination and then decreased at the late stage, which was similar to the expressional patterns of glycolysis-related enzymes. However, tricarboxylic acid cycle-related enzymes only increased at the later stage. It was also found that sucrose might be an important intermediate for the biosynthesis of starch in embryos. Furthermore, gel-based proteomic analysis of the dissected endosperm showed that the biological processes in the endosperm were heavily regulated by the embryo. This study could provide some new insights into the distinct roles of the embryo and endosperm in rice seed germination.